1. Field
The present invention relates to a multi-component catalytic system that can be used for the cis-1,4 stereospecific polymerization of conjugated dienes. More particularly, the present invention relates to a catalytic system comprising a rare-earth complex, and also to a process for preparing said catalytic system and the use of this catalytic system for the preparation of diene elastomers having a high content of cis-1,4 linkages.
2. Description of Related Embodiments
Boisson et al., Macromol. Chem. Phys., 1999, 200, 1163-1166 and Monteil et al., Polymer Int., 2004, 53, 576-581 describe the use of multicomponent catalytic systems for the polymerization of butadiene comprising:                a neodymium salt of tris(amide) type of formula Nd[N(TMS)2]3, namely neodymium tris[N,N-bis(trimethylsilyl)amide],        an alkylaluminium of formula Al(i-Bu)3, and        an alkylaluminium halide, formula AlEt2Cl.        
These catalysts are prepared in situ and used for the polymerization of butadiene and for the copolymerization of butadiene and styrene. The molecular weight distribution of the polybutadienes synthesized is broad and multimodal, which is the result of a poor control of the polymerization. Moreover, such a molecular weight distribution may be detrimental in view of certain applications of the diene elastomer prepared, such as in particular in tyres for motor vehicles.
In the same spirit, Patent Application WO2003033545 describes multicomponent catalytic systems for the polymerization of conjugated dienes, some of which are based on a neodymium salt of tris(amide) type, for example of formula Nd[N(TMS)2]3, and on a co-catalyst. In the exemplary embodiments the active species formed by reaction of the neodymium salt and the co-catalyst is of cationic type since the co-catalyst systematically comprises an aluminoxane (modified methylaluminoxane—MMAO or isobutylaluminoxane—IBAO). Optionally, an alkylaluminium halide or a borane (B(C6F5)3) is added to the catalytic system illustrated.
Document U.S. Pat. No. 3,297,667 also describes multicomponent catalytic systems based on                (i) a rare-earth compound,        (ii) a bidentate organic ligand,        (iii) a halide, and        (iv) an alkylaluminium.        
The catalytic system is prepared by reaction of (i) with (ii) which results in a chelated isolated species of the rare-earth metal. This species is then brought into contact with the halide and the alkylaluminium. As compound (i), the process for preparing catalytic systems of this document essentially uses cerium chloride. Yet, rare-earth chlorides may have an unsatisfactory reactivity with certain organic ligands (ii) present in the reaction media, due to a low solubility of the chlorides in these media.
Furthermore, Patent Document U.S. Pat. No. 7,300,903 B2 describes a process for polymerizing olefins using a catalytic system based on:                an isolated compound based on a transition metal corresponding to the Formula (I) below, for the complete definition of which reference should be made to the text of the patent,        
                                    in which M is a metal belonging to groups 3 to 11 of the Periodic Table of the Elements,                        at least one compound selected from:                    an organometallic compound,            an organoaluminium compound containing an oxygen atom,            a compound that can react with the compound based on a transition metal in order to form an ion pair.                        
According to the patent, in the Formula I, the metal M that is favoured and described in the examples belongs mainly to group 4. It is titanium, zirconium or optionally hafnium. All the examples of this patent relating to ethylene and butadiene copolymerization tests were carried out starting from catalytic systems based on titanium or zirconium. Depending on the experimental conditions and the nature of the catalytic system, these tests made it possible, in particular to obtain copolymers where the proportion of butadiene is very low and reaches at most 6.6 mol % (Examples 150 to 157). Although no butadiene homopolymerization test is described, the poor capacity of the catalytic systems of this patent to insert this monomer implies that these systems are unsuitable for the cis-1,4 stereospecific polymerization of butadiene.
In view of an application as a tyre for motor vehicles and more particularly in the tread, it is essential to have diene elastomers of specific microstructure, in particular polybutadiene having a high content of cis-1,4 linkages. Furthermore, a controlled, or even narrow, molecular weight distribution makes it possible to adjust the macrostructural characteristics of the elastomer as best possible as a function of the properties desired during use in the tread. Therefore catalytic systems and polymerization processes that make it possible to reproducibly obtain diene elastomers having specific microstructure and macrostructure characteristics, and in particular diene elastomers having a high content of cis-1,4 linkages and having a controlled molecular weight distribution are constantly being sought.